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46th Lunar and Planetary Science Conference (2015)
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SECONDARY MINERALS IN THE NAKHLITE METEORITE YAMATO 000593: DISTINGUISHING
MARTIAN FROM TERRESTRIAL ALTERATION PRODUCTS. H. Breton1, M. R. Lee1, and D. F. Mark2
1
School of Geographical and Earth Sciences, University of Glasgow, University Ave, Glasgow, Lanarkshire G12
8QQ, UK ([email protected]), 2Scottish Universities Environmental Research Center, Rankine Ave,
Scottish Enterprise Technology Park, East Kilbride G75 0QF, UK
Introduction: The nakhlites are olivine-bearing
clinopyroxenites that formed in a Martian lava flow or
shallow intrusion 1.3 Ga ago [1, 2]. They are scientifically extremely valuable because they interacted with
water-bearing fluids on Mars [3]. Fluid-rock interactions led to the precipitation of secondary minerals,
many of which are hydrous. The secondary minerals
consist in a mixture of poorly crystalline smectitic material and Fe-oxide, collectively called “iddingsite”, but
also carbonate and sulphate [4]. The proportion, chemistry and habit of the secondary minerals vary between
members of the Nakhlite group, which is thought to
reflect compositional variation of the fluid within the
Martian crust [5]. However, some secondary minerals
are quite similar to terrestrial alteration products and
thus the chemical and textural variations could also
reflect terrestrial contamination (deposition or exchange). Identifying the origin of the secondary minerals is not straightforward but essential to unravel the
Martian fluid chemistry and conditions.
Yamato 000593 (Y-000593) is a nakhlite meteorite
that was discovered in Antarctica near the Yamato
Mountains by the Japanesse Antarctic Research Expedition in 2000-2001 [6]. Most of the meteorite is covered by a black shiny fusion crust but it also has deep
erosion features in its underside that probably formed
by freeze- thaw cycles. As in most other Nakhlites, Y
000593 contains iddingsite-like alteration products
believed to have been formed on Mars because they
have devolatilization features at the vicinity of the fusion crust [7]. Additional evidence of Martian aqueous
alteration is the presence of laihunite, a high temperature oxidative alteration product of fayalitic olivine [8].
The secondary minerals in Y-000593 can provide a
powerful insight into the Martian hydrosphere from
high to low temperature environments with implications for the origin, cycling, and availability of water
on Mars. However, it is highly likely that some secondary minerals have formed on Earth which can biased our understanding of the Martian groundwater
chemistry. With this in mind, we are trying to identify
all the different secondary minerals and document their
spatial and textural relations, their mineralogy and
chemistry to better constrain their possible origin and
the impact that terrestrial fluids may have had on the
Martian alteration products.
Methods: A thin section of Y-000593 was studied
using a Carl Zeiss Sigma field-emission SEM equipped
with an Oxford Instruments Aztec microanalysis system at the University of Glasgow. Chemical and mineralogical identification within the secondary minerals
were obtained through backscattered electron (BSE)
imaging and energy dispersive spectroscopy (EDS)
mapping and quantitative microanalysis.
Results and discussions: Y-000593 is an unbrecciated cumulate rock whose mineralogy is similar to
other nakhlites: a predominance of augite and minor
olivine phenocrysts surrounded by a microcrystalline
mesostasis [9]. Although augite is the most abundant
mineral, it contains only rare traces of secondary minerals which include Ca-sulphate filling fractures and
phyllosilicate replacement (recognized by their habit
and crystallinity). Unlike augite, olivine phenocrysts
have a variety of alteration textures and products (Fig.
1). Alteration products commonly enclose symplectites
suggesting that they formed after the augite-magnetite
intergrowths and by selective replacement of olivine.
Most of the alteration products are confined within
veins few µm-thick cross-cutting olivine.
Outside of these veins, olivine is discontinuously
replaced by an alteration product, already identified as
laihunite [8], which has a characteristic fine-scale
channel-like micro-texture (Fig. 1). The distribution of
the laihunite is strongly controlled by grain boundaries,
lamellae symplectites and veins from which it seems to
have developed.
X-ray elemental imaging of the laihunite reveals no
major compositional difference relative to unaltered
olivine. It shows only a slight depletion in Fe and enrichment in O suggesting partial dissolution of olivine
by water-bearing fluids. No changes in Mg content
have been detected; however laihunite shows traces of
S most probably present as adsorbed ions within the
channels.
Olivine-hosted iddingsite veins are zoned with a
complex secondary mineral assemblage. Alteration
products present along vein axes do not show evident
crystals or grains at the SEM scale. They are enriched
in Si, Mg, and O with lower concentration in Fe and
rare Al. This material is commonly surrounded by a
discontinuous, fine layer of Fe-Mn-oxide. Both mineral
phases represent the Martian “iddingsite” common to
46th Lunar and Planetary Science Conference (2015)
most nakhlites. The “iddingsite” has largely been replaced coarser minerals enriched in Fe, Mn, Al, S, P,
and Cl but poorer in Si and Mg. The proportions of
these components varies at the (sub)µm-scale. Finally,
the different materials are cross-cut by a K-Fe-Sbearing mineral phase (jarosite?). The presence of different mineral phases in cross-cutting and overgrowing
relationships points toward multiple episodes of secondary mineral deposition.
2010.pdf
The other mesostasis alteration product occurs
mostly within the phyllosilicates. It is highly porous,
fibrous and composed mostly of Fe and Mn, with low
concentration of Si and variable P, S, and Cl. The occurrence of Si and the fibrous texture suggest the presence of a phyllosilicate mineral. The ionic components
may represent either a co-precipitation of salts with the
phyllosilicate mineral or a substitution of OH molecules by anions. This alteration product is similar in
composition (albeit poorer in Si) and texture to that
overgrowing the olivine-hosted “iddingsite”, suggesting that they may have precipitated from a same fluid
but with a micro-environmental control on the fluid
chemistry. It was most probably the last to crystallize
and may be of terrestrial origin. The occurrence of anions may be the result of dissolution of primary minerals such as pyrrhotite and apatite.
Fig. 1: BSE image of olivine-hosted laihunite and alteration veins with iddingsite and S-P-Cl-bearing mineral. Laihunite develops along narrow fractures from
which it replaces olivine.
The mesostasis also contains significant amounts of
alteration products. They are not restricted to veins but
rather form concentrated masses replacing different
igneous phases especially pyrrhotite, Ti-magnetite,
augite, and plagioclase feldspar (Fig. 2, 3). The alteration products are composed of a fine mixture of two
different types of secondary mineral. One forms layers
surrounding and replacing primary minerals although it
has never been found associated with altered olivine.
This alteration product is fibrous and composed of Si,
Fe, Mn, Mg, and Al. Traces of P, Cl and S have been
detected although it may reflect mixed analysis with the
other alteration product. Both its microstructure and
chemical composition suggest that it is a phyllosilicate.
Fig. 2: Two different alteration products (1 and 2) replacing an unusually large grain of pyrrhotite
Fig. 3: Zoned phyllosilicates replacing mesostasis and
broken augite phenocrysts.
Conclusions: Martian alteration products in Y000593 include laihunite and iddingsite, both strongly
associated with olivine phenocrysts. Terrestrial alteration products possibly include the phyllosilicates together with an anion-bearing mineral (S, P, and Cl).
These latter components do not form well-defined
veins but commonly replace primary material and occur in shocked areas. The relative timing of the alteration material containing the anionic constituents S, P,
and Cl is difficult to constrain, but may represent the
latest alteration products that formed by terrestrial processes.
References: [1] Lentz F. R. C. Et al. (1999) Meteoritics & Planet. Sci., 34, 919-932. [2] Nakamura N. Et
al. (1982) Geochim. Cosmochim. Acta, 46, 1555-1573.
[3] Gooding J.L. (1997) Icarus, 99, 28-41. [4] Bridges
J.C. et al. (2001) Space Sci. Rev., 96, 365-392. [5]
Tosca N. J. and McLennan S. M. (2006) EPSL, 241,
21-31. [6] Misawa et al. (2003) Antarct. Meteorite
Res., 16, 1-12. [7] Imae et al. (2003) Antarct. Meteorite Res., 16, 13-33. [8] Noguchi et al (2009) J. of Geophys. Res., 114, 1-13. [9] Imae et al. (2005) Meteoritics & Planet. Sci., 40, 1581-1598.